Black-eyed Susan usually is an annual herb, unbranched, very hairy. Flowerheads solitary or in loose, open clusters, terminal on the stalk, to 4 inches across. Ray flowers 8–21, rich yellow or orangish, slender. Disk hemispherical, becoming egg-shaped, deep brown to purple-brown. Blooms May–October. Leaves hairy, sessile except a few basal leaves, lanceolate, sometimes with fine teeth.
Similar species: There are 9 Rudbeckia species in Missouri. Four of the most commonly encountered are brown-eyed Susan (R. triloba, to 5 feet tall; small flowerheads to 1 inch across; leaves with coarse teeth); wild goldenglow (R. laciniata, to 9 feet tall; green disk; 6–10 yellow rays; leaves with 3–7 deeply cut lobes); Missouri black-eyed Susan (R. missouriensis, like R. hirta but smaller; very hairy; with all but lowest leaves linear); and sweet coneflower (R. subtomentosa, a hairy plant to 6 feet tall; 12–20 yellow rays per head; 3-lobed lower leaves).